Ferronickel and process for producing raw material for ferronickel smelting

ABSTRACT

The present invention provides a process that is useful in producing ferronickel having a high Ni content at low cost with high efficiency and reproducibility even if a low-grade feedstock containing nickel oxide is used. In particular, a feedstock containing nickel oxide and iron oxide is mixed with a carbonaceous reductant, the mixture is formed into agglomerates with an agglomerator, and the agglomerates are heated and reduced in a moving hearth furnace, whereby reduced agglomerates in which the Ni metallization degree is 40% or more and the Fe metallization degree is at least 15% less than the Ni metallization degree are prepared by adjusting the retention time of the agglomerates placed in the moving hearth furnace. The reduced agglomerates, in which the Ni component has been primarily reduced as compared with the Fe component, are smelted in a smelting furnace, whereby ferronickel having a high Ni content is obtained.

TECHNICAL FIELD

The present invention relates to processes for producing ferronickel andparticularly relates to a process for producing ferronickel or afeedstock for ferronickel production with high efficiency usinglow-grade nickel oxide ore.

BACKGROUND ART

Examples of a process, currently used in our country, for producingferronickel from nickel oxide ore include electric furnace processes andthe Krupp-Ren process, and the electric furnace processes arecategorized into a complete reduction process and a selective reductionprocess (see The Iron and Steel Institute of Japan, “TEKKO BINRAN fourthedition”, Volume 2, Chapter 7, Section 3, Subsection 4, published by TheIron and Steel Institute of Japan, published on Jul. 30, 2002).

In the complete reduction process, which is one of the electric furnaceprocesses, nickel oxide ore is mixed with coal, the mixture is calcinedin a rotary kiln, and the resulting mixture is melted and reduced in anelectric furnace, whereby ferronickel is produced In the selectivereduction process, nickel oxide ore is mixed with coal, the mixture ispreliminarily reduced in a rotary kiln, and the resulting mixture ismelted and further reduction is completed in an electric furnace,whereby ferronickel is produced.

In the Krupp-Ren process, nickel oxide ore is mixed with anthracite, themixture is pressed into briquettes, and the briquettes are heated andreduced in a rotary kiln, whereby a semi-solid containing luppe (metal)and slag are prepared. The semi-solid is water-granulated, and the luppeis then isolated and recovered from the semi-solid by a magneticseparation process or a flotation process, whereby ferronickel isproduced.

In the above processes, rotary kilns are used and there are manyproblems, described below, characteristic of such rotary kilns. That is,since the rotary kilns are based on the principle that raw materials aremoved by rolling motion, a large amount of dust is generated; hence,there is a problem in that a dam ring is apt to be formed. In order toprevent the dam ring from being formed, a technique of limiting thecontent of slag in a feedstock has been proposed (see, for example,Japanese Examined Patent Application Publication No. 48-43766, secondpage); however, there is a problem in that the degree of freedom infeedstock selection is low. Furthermore, in order to operate a kilnwithout trouble, the outlet temperature must be relatively low; hence,the kiln must be have a large size so as to provide a long retentiontime (see Japanese Unexamined Patent Application Publication No.9-291319, second page). Therefore, there is a problem in that the fuelconsumption is high because the kiln has a large surface area and theheat release is therefore high.

Furthermore, in the above processes, since nickel oxide and iron oxidein nickel oxide ore are reduced into metals at the same time but onlythe nickel oxide cannot be reduced into a metal, there is a problem inthat ferronickel having a high Ni content cannot be produced from orehaving a low Ni content. In order to solve that problem, techniquesdescribed below have been proposed.

One of the techniques is similar to the Krupp-Ren process. In thistechnique, nickel oxide ore is pretreated, the resulting ore is reducedin a firing furnace while the ore is in a semi-molten state, andmetallic Fe and metallic Ni are recovered from the ore, wherebyferronickel is produced. In the step of reducing the ore in the firingfurnace while the ore is in a semi-molten state, an Fe component and anNi component are reduced in a strongly reducing atmosphere and only theFe component is then oxidized in a strongly oxidizing atmosphere,whereby the content of Ni in the luppe is relatively increased andferronickel having a high Ni content is produced (see JapaneseUnexamined Patent Application Publication No. 5-186838).

The other one is also similar to the Krupp-Ren process. In thistechnique, nickel oxide ore is pretreated, the resulting ore is reducedin a firing furnace while the ore is in a semi-molten state, andmetallic Fe and metallic Ni are recovered from the ore, wherebyferronickel is produced. In the step of reducing the ore in the firingfurnace while the ore is in a semi-molten state, a predetermined amountof additional coal necessary to reduce (metallize) an Ni component andan Fe component to a desired level is divided into several portions,which are fed into the furnace intermittently or continuously, wherebythe content of Ni in the luppe is relatively increased and ferronickelhaving a high Ni content is produced (see Japanese Unexamined PatentApplication Publication No. 5-247581).

If an attempt to use the rotary kiln as a firing furnace is made tocommercialize the above techniques (Japanese Unexamined PatentApplication Publication Nos. 5-186838 and 5-247581) for producingferronickel from the low-Ni content ore, the attempt is not successfuldue to the structure thereof. This is because it is necessary for thetechniques to feed a gas or solid material into the kiln though a sidewall of the kiln that is usually rotating and the structure of the kilnis therefore complicated; hence, trouble frequently occurs during theoperation and the plant cost is high.

Even if the structural problem of the kiln is solved, the followingproblems that are characteristic of the rotary kiln still remain: alarge amount of dust is generated, the dam ring is formed in the kiln,the plant area is large, and a large amount of heat is released fromwalls, which causes an increase in fuel consumption.

DISCLOSURE OF INVENTION

Under the circumstances described above, it is an object of the presentinvention to provide a process for producing ferronickel having a highNi content from low-grade nickel oxide ore (a feedstock containingnickel oxide) at low cost with high efficiency without trouble.

According to a first aspect of the present invention, a process forproducing ferronickel includes a mixing step of mixing a feedstockcontaining nickel oxide and iron oxide with a carbonaceous reductant toprepare a mixture, a reducing step of heating and reducing the mixturein a moving hearth furnace to prepare a reduced mixture, and a smeltingstep of smelting the reduced mixture in a smelting furnace to prepareferronickel.

In the process, since the moving hearth furnace is used to heat andreduce the mixture, the amount of dust generated is greatly decreasedand the dam ring caused by the dust deposited on furnace walls isprevented from being formed. Thus, in order to prevent the dam ring frombeing formed, the slag content of the feedstock need not be adjusted;hence, the degree of freedom in feedstock selection is high. Since theretention time of the mixture placed in the moving hearth furnace isuniform, a large size apparatus such as a rotary kiln is not necessaryand the plant is compact; hence, the plant area is small and the heatrelease is low.

In the process, the retention time of the mixture placed in the movinghearth furnace is preferably adjusted such that the Ni metallizationdegree of the reduced mixture is 40% or more (preferably 85% or more)and the Fe metallization degree of the reduced mixture is at least 15%less than the Ni metallization degree thereof.

When the Fe metallization degree of the reduced mixture is adjusted to avalue that is at least 15% less than the Ni metallization degree bycontrolling the retention time of the mixture placed in the movinghearth furnace, nickel oxide in ore having a low Ni content is primarilymetallized but iron oxide therein is slowly metallized; hence,ferronickel having a high Ni content can be readily produced with highefficiency by smelting the reduced mixture in the smelting furnace Whenthe Ni metallization degree of the reduced mixture is 40% or more, theamount of heat necessary to reduce nickel oxide remaining in the reducedmixture in the smelting furnace is small; hence, there is an advantagein that the energy consumption of the smelting furnace can be reduced.The Ni metallization degree and the Fe metallization degree are definedas follows:

Ni metallization degree (%)=[(metallic Ni content (% by mass))/(total Nicontent (% by mass))]×100

Fe metallization degree (%)=[(metallic Fe content (% by mass))/(total Fecontent (% by mass))]×100

The process preferably further includes a reduced mixture-retaining stepof cooling the reduced mixture to a temperature ranging from 450° C. to1100° C. in the moving hearth furnace to maintain the reduced mixture atthat temperature for 17 seconds or more or discharging the reducedmixture from the moving hearth furnace to place the reduced mixture inanother vessel to cool the reduced mixture to a temperature ranging from450° C. to 1100C. in the vessel to maintain the reduced mixture at thattemperature for 17 seconds or more.

When the reduced mixture is maintained at a temperature ranging from450° C. to 1100° C. for a predetermined time, a reaction in which nickeloxide contained in the reduced mixture is reduced with metallic iron andmetallic nickel and iron oxide are thereby formed is promoted, thereaction being expressed as NiO+Fe→Ni+FeO; hence, the Ni metallizationdegree can be increased but the Fe metallization degree can bedecreased. That is, Ni can be more primarily reduced.

According to a second aspect of the present invention, a process forproducing ferronickel includes a mixing step of mixing a feedstockcontaining nickel oxide and iron oxide with a carbonaceous reductant toprepare a mixture, a reducing and melting step of heating and reducingthe mixture in a moving hearth furnace to prepare a reduced mixture inwhich the Ni metallization degree is 40% or more (preferably 85% ormore) and the Fe metallization degree is at least 15% less than the Nimetallization degree to heat and melt the reduced mixture to prepare areduced melt, a solidifying step of cooling the reduced melt in themoving hearth furnace to prepare a reduced solid or discharging thereduced melt from the moving hearth furnace to cool the reduced melt toprepare a reduced solid, and a separating step of separating the reducedmelt into metal and slag to prepare ferronickel.

According to this process, since ferronickel having a high Ni contentcan be produced by reducing and melting ore having a low Ni content onlyin the moving hearth furnace, no melting furnace is used; hence, plantcost and the energy consumption can be greatly decreased

Furthermore, a process for producing a feedstock for ferronickelproduction according to the present invention includes a mixing step ofmixing a feedstock containing nickel oxide and iron oxide with acarbonaceous reductant to prepare a mixture and a reducing and meltingstep of heating and reducing the mixture in a moving hearth furnace toprepare a ferronickel feedstock in which the Ni metallization degree is40% or more (preferably 85% or more) and the Fe metallization degree isat least 15% less than the Ni metallization degree.

In the feedstock-producing step, as well as the process of the firstaspect, since the moving hearth furnace, which is of a stationary type,is used to heat and reduce the mixture, the amount of dust generated isgreatly decreased and the dam ring caused by the dust deposited onfurnace walls is prevented from being formed. Thus, in order to preventthe dam ring from being formed, the slag content of the feedstock neednot be adjusted; hence, the degree of freedom in feedstock selection ishigh. Since the retention time of the mixture placed in the movinghearth furnace is uniform, a large size apparatus such as a rotary kilnis not necessary and the plant is compact; hence, the plant area issmall and the heat release is low. When the Fe metallization degree isadjusted to a value that is at least 15% less than the Ni metallizationdegree by controlling the retention time of the mixture placed in themoving hearth furnace, nickel oxide in ore having a low Ni content isprimarily metallized but iron oxide therein is slowly metallized; hence,a feedstock for producing ferronickel having a high Ni content can bereadily produced with high efficiency. Furthermore, when the Nimetallization degree of the reduced mixture is 40% or more (preferably85% or more), the amount of heat necessary to reduce nickel oxide in thesmelting furnace in a subsequent step is small; hence, the energyconsumption can be decreased.

Embodiments of the present invention will now be described in detailwith reference to the accompanying drawings.

FIRST EMBODIMENT

FIG. 1 shows steps for producing ferronickel according to an embodimentof the present invention. In the figure, reference numeral 1 representsa feedstock containing nickel oxide and iron oxide (hereinafter simplyreferred to as “a feedstock”), reference numeral 2 represents acarbonaceous reductant, reference numeral 3 represents an agglomerator,reference numeral 4 represents agglomerates (mixture), reference numeral5 represents a moving hearth furnace, reference numeral 6 representsreduced agglomerates (reduced mixture), reference numeral 7 represents asmelting furnace, reference numeral 8 represents metal (ferronickel),and reference numeral 9 represents slag.

Examples of the feedstock 1 containing nickel oxide and iron oxideinclude a nickel oxide ore and a residue, generated in ferronickel- ornickel-manufacturing steps, such as kiln dust generated fromferronickel-manufacturing plants. Examples of the nickel oxide oreinclude garnierite usually used and low-grade nickel ores such asnickel-containing laterite and limonite. Those ores and residue may beused in combination. Since a rotary kiln is not used but the movinghearth furnace 5 that is of a stationary type is used, a dam ring isprevented from being formed and there are no limitations on slagcomponents; hence, the degree of freedom in feedstock selection is high.When the feedstock 1 contains a large amount of water, the feedstock 1is preferably dried in advance. The dryness of the feedstock 1 may bedetermined depending on the type of a mixing means (the agglomerator 3is used in this embodiment) used in a subsequent mixing step. Thecarbonaceous reductant 2 contains fixed carbon, and any reductant suchas coal, coke, charcoal, waste toner, or carbonized biomass may be used.Those materials may be used in combination.

The content of the carbonaceous reductant 2 in the agglomerates(mixture) 4 may be determined based on the amount of carbon necessary toreduce nickel oxide and iron oxide contained in the feedstock 1 in themoving hearth furnace 5, the amount of carbon consumed in reducingnickel oxide remaining in reduced agglomerates (reduced mixture) 6 inthe smelting furnace 6, and the amount of carbon remaining inferronickel.

[Mixing Step]: A mixer, which is not shown, is preferably used to mixthe feedstock 1 and the carbonaceous reductant 2. The mixture may bedirectly fed into the moving hearth furnace 5; however, the mixture ispreferably agglomerated with the agglomerator 3. This is because theagglomeration reduces the amount of dust generated from the movinghearth furnace 5 and the smelting furnace 7 and enhances theheat-transfer efficiency of the agglomerates (mixture) 4 placed in themoving hearth furnace 5 to increase the reduction rate. The agglomerates(mixture) 4 may contain an auxiliary feedstock such as flux. Examples ofthe agglomerator 3 include a compression molding machine such as abriquette press, a rotary pelletizer such as a disk pelletizer, and anextruder. When the content of water in the obtained agglomerates(mixture) 4 is high, the agglomerates (mixture) 4 may be dried beforethey are fed into the moving hearth furnace 5.

[Reducing Step]: The agglomerates (mixture) 4 are fed into the movinghearth furnace 5 and then heated and reduced at an atmospherictemperature ranging from 1000° C. to 1400C.

Examples of the moving hearth furnace 5 include known moving hearthfurnaces having at least one bed on which the agglomerates (mixture) 4are placed and which horizontally moves in the furnace and also have aheating means for reduction or the like and there are no limitations onsuch moving hearth furnaces. Examples of the moving hearth furnacesinclude a rotary hearth furnace, a straight furnace, and a multi-hearthfurnace. In those moving hearth furnaces, the amount of dust and thelike generated is small because heating objects (agglomerates are hereinused) are kept stationary. Furthermore, plant cost is relatively low,operation troubles hardly occurs, and the heat release is small becausethe surface are is less than that of rotary kilns; hence, the reductionefficiency is high.

The retention time of which the agglomerates (mixture) 4 heated to atemperature ranging from 1000° C. to 1400° C. is preferably adjustedwithin a range of 3 to 20 minutes such that the relationship between theNi metallization degree and the Fe metallization degree is satisfied asdescribed below. That is, in the moving hearth furnace 5, theagglomerates (mixture) 4 are reduced such that the Ni metallizationdegree of the agglomerates (mixture) 4 is 40% or more (more preferably50% or more and further more preferably 85% or more) and the Femetallization degree of the agglomerates (mixture) 4 is at least 15%(more preferably at least 20%) less than the Ni metallization degreethereof. The Fe metallization degree is determined based on the Nicontent and Fe content of the feedstock 1 and the target content of Niin the ferronickel 8, which is a product. In order to produce theferronickel 8 having an Ni content of, for example, 16%, the Nimetallization degree and Fe metallization degree shown in FIG. 2 must beachieved depending on the kinds of nickel oxide ore (see Table 1) usedfor the feedstock 1. For the standard ore, except for the high-gradeore, the Fe metallization degree must be 15% to 20% less than the Nimetallization degree. For the low-grade ores, the Fe metallizationdegree must be less than the above. TABLE 1 Content Kinds of Nickel (%by mass) Oxide Ore Ni Fe Standard Ore 2.4 14.7 High-grade Ore 2.5 9.8Low-grade Ore 1 1.7 15.7 Low-grade Ore 2 1.3 34.5

FIG. 3 shows the target Fe metallization degree determined based on theNi content and Fe content of the feedstock 1. The target Femetallization degree is in proportion to the Ni content of the feedstock1 but is in inverse proportion to the Fe content of the feedstock 1. InFIG. 3, the content of Ni in the ferronickel 8 is 16% and the Nimetallization degree of the reduced agglomerates (reduced mixture) 6 is90%. In order to increase the Ni content of the ferronickel 8 from 16%to, for example, 20%, the Fe metallization degree must be furtherdecreased.

The Ni metallization degree and the Fe metallization degree may beadjusted by varying the heating temperature and the retention time basedon a difference in reduction rate between both components. In general,the Ni component is more readily reduced (metallized) than the Fecomponent because the affinity of Ni to oxygen is less than that of Fe.Although the difference between the Ni metallization degree and the Femetallization degree is small when the retention time is short, anincrease in the retention time allows the Ni metallization degree andthe Fe metallization degree to approach 100% if there is a sufficientamount of a reductant. Furthermore, an increase in heating temperaturepromotes the reduction of FeO, whereby the difference between the Nimetallization degree and the Fe metallization degree is decreased (seeFIGS. 5 and 6). Therefore, the heating temperature and the retentiontime are adjusted depending on the reduction efficiency such that the Nimetallization degree slightly exceeds the Fe metallization degree.

On the other hand, as described in examples described below, a decreasein the surplus of carbon (%) causes a decrease in the Fe metallizationdegree (see FIG. 8) and an increase in the strength (for example, thecrushing strength) of reduced agglomerates, whereby the agglomerates canbe readily handled and the yield of a smelting operation is increased.Thus, the surplus of carbon is preferably 0% or less, more preferably−2% or less, and first more preferably −4% or less.

The Ni metallization degree and Fe metallization degree of the reducedagglomerates (reduced mixture) 6 can be controlled by varying thecontent of the carbonaceous reductant 2 in the agglomerates (mixture) 4and can also be controlled by varying the retention time. When themetallization degrees are controlled by varying the content and theretention time, the degree of freedom in feedstock selection and the Nicontent of the ferronickel 8 can be increased.

The reduced agglomerates (reduced mixture) 6 treated in the movinghearth furnace 5 are usually cooled to about 1000° C. with a radiantcooler or coolant sprayer placed in the moving hearth furnace 5 and thendischarged with a discharging unit.

[Step of Retaining Reduced Mixture]: Unreduced nickel oxide is reducedby metallic Fe while the reduced agglomerates (reduced mixture) 6 arecooled. In particular, a reaction expressed as NiO+Fe→Ni+FeO occurs tocause an increase in the Ni metallization degree but a decrease in Femetallization degree, whereby the Ni component in the reducedagglomerates (reduced mixture) 6 is primarily reduced. In order toactively use the reaction, the reduced agglomerates (reduced mixture) 6are preferably cooled to a temperature ranging from 450° C. to 1100° C.and maintained at that temperature for 17 seconds or more while thereduced agglomerates (reduced mixture) 6 are placed in the moving hearthfurnace 5 or placed in another vessel, which is not shown, after thereduced agglomerates (reduced mixture) 6 are discharged from the movinghearth furnace 5. The reason for setting the lower limit of thetemperature to 450° C. is that the reaction rate is too low and theadvantage is therefore low when the temperature is less than 450° C. Thelower limit is more preferably 650° C. In contrast, the reason forsetting the upper limit of the temperature to 1100° C. is that ironoxide and the carbonaceous reductant remaining in the reducedagglomerates (reduced mixture) 6 react each other and the chainreactions expressed as FeO+CO→Fe+C₀₂ and C₀₂+C→2CO are promoted when thetemperature is more than 1100° C., whereby the Fe metallization degreeis increased. The upper limit is more preferably 1000° C.

The reason for setting the lower limit of the retaining time to 17seconds during which the reduced agglomerates (reduced mixture) 6 aremaintained at a temperature within the above range is described below.The following equation has been obtained by formulating the correlationbetween the reduction time and Ni metallization degree (see FIG. 6)obtained from a reduction experiment (an atmospheric temperature of1300° C.) performed in an example described below:MetNi=83.9×[1−exp(−t/46)]+15.3wherein MetNi represents the Ni metallization degree (%) and trepresents the reduction time (s).

According to the above equation, it takes 17 seconds to reduce 30% ofunreduced nickel-oxide (NiO) contained in the reduced agglomerates(reduced mixture) 6 into metallic Ni; hence, the lower limit of theretaining time is specified as 17 seconds. Considering that the upperlimit (1100° C.) of the cooling temperature is lower than theatmospheric temperature (1300° C.) of the reduction experiment, thelower limit of the retaining time is preferably set to 20 seconds, whichis slightly longer than 17 seconds. Furthermore, according to theequation, it takes 32 seconds to reduce 50% of unreduced nickel oxide(NiO) contained in the reduced agglomerates (reduced mixture) 6 intometallic Ni; hence, the lower limit of the retaining time is morepreferably 32 seconds and further more preferably 40 seconds. As long asthe reduced agglomerates (reduced mixture) 6 are maintained at atemperature within the above range, the retaining time may include thetime elapsed during the transfer of the reduced agglomerates (reducedmixture) 6 that is discharged from the moving hearth furnace 5 or thevessel and then fed into the smelting furnace 7.

[Melting Step]: The reduced agglomerates (reduced mixture) 6 dischargedfrom the moving hearth furnace 5 or the vessel are preferably fed intothe smelting furnace 7 directly in such a manner that the reducedagglomerates (reduced mixture) 6 are not further cooled but maintainedat a high temperature The smelting furnace 7 may be directly connectedto an outlet of the moving hearth furnace 5 or the vessel with a chuteplaced therebetween. The reduced agglomerates (reduced mixture) 6 may befed into the smelting furnace 7 using a conveying unit such as aconveyer or using a container for temporarily storing the reducedagglomerates (reduced mixture) 6. When the smelting furnace 7 is notplaced close to the moving hearth furnace 5 or is not operated, thereduced agglomerates (reduced mixture) 6 may be-cooled to atmospherictemperature and then treated as a semi-product (feedstock forferronickel production) during the storage or the transportation.Alternatively, the reduced agglomerates (reduced mixture) 6 may behot-briquetted to reduce the surface area in such a manner that thereduced agglomerates (reduced mixture) 6 are not cooled but maintainedat a high temperature, whereby the surface area thereof is deceased. Thehot-briquetted reduced agglomerates (reduced mixture) 6 are cooled andthen treated as a semi-product (feedstock for ferronickel production)having high re-oxidation resistance during the storage or thetransportation.

Examples of the smelting furnace 7 include an electric furnace. When theelectric furnace is used, the content of carbon in molten metal, thevoltage of the electric furnace, the positions of electrodes arranged inthe electric furnace, the amount of oxygen, and the amount of agitationgas are preferably adjusted such that the nickel yield is increased andthe reduction of iron is suppressed. A smelting furnace using fossilenergy such as coal, fuel oil, natural gas may be used. The flux or thelike may be fed into the smelting furnace 7 according to needs and thereduced agglomerates (reduced mixture) 6 are melted at a hightemperature ranging from 1400° C. to 1700° C., whereby the reducedagglomerates (reduced mixture) 6 are separated into the matal 8 and theslag 9. The metal 8, which corresponds to the ferronickel 8, iswithdrawn and then additionally refined according to needs, wherebycommercial ferronickel is produced. The slag 9 can be used for concreteaggregates.

SECOND EMBODIMENT

FIG. 4 shows steps of producing ferronickel according to anotherembodiment of the present invention. In the figure, reference numeral 11represents a feedstock containing nickel oxide and iron oxide(hereinafter simply referred to as “a feedstock”), reference numeral 12represents a carbonaceous reductant; reference numeral 13 represents anagglomerator, reference numeral 14 represents agglomerates (mixture),reference numeral 15 represents a moving hearth furnace, referencenumeral 16 represents a reduced solid, reference numeral 17 represents ascreen, reference numeral 18 represents metal (ferronickel), andreference numeral 19 represents slag.

In the second embodiment, the feedstock 11, the carbonaceous reductant12, the agglomerator 13, and the agglomerates (mixture) 14 are the sameas the feedstock 1, carbonaceous reductant 2, agglomerator 3, andagglomerates (mixture) 4 of the first embodiment, respectively, and amixing step is also the same as that of the first embodiment; hence, thedescription is omitted.

[Reducing/Melting Step]: The agglomerates (mixture) 14 are fed into themoving hearth furnace 15 and then heated and reduced at an atmospherictemperature ranging from 1000° C. to 1400° C. According to the sameconcept as that described in the first embodiment, the retention time ofthe agglomerates (mixture) 14 heated to a temperature ranging from 1000°C. to 1400° C. may be adjusted within a range of 3 to 20 minutes suchthat the relationship between the Ni metallization degree and the Femetallization degree satisfies conditions described below. That is, inthe above temperature range, the agglomerates (mixture) 14 are reducedin reduced agglomerates (reduced mixture) such that the Ni metallizationdegree of the reduced agglomerates (reduced mixture) is 40% or more(more preferably 50% or more, further more preferably 85% or more, mostpreferably 90% or more) and the Fe metallization degree thereof is atleast 15% (more preferably at least 20%) less than the Ni metallizationdegree. Subsequently, the reduced agglomerates (reduced mixture) areheated and melted at an atmospheric temperature ranging from 1100° C. to1500° C., which is higher than that described above, in the movinghearth furnace 15, whereby reduced melt is prepared. The retention timeof the reduced agglomerates (reduced mixture) heated to an atmospherictemperature ranging from 1100° C. to 1500° C. may be adjusted within arange of 0.5 to 10 minutes such that the reduced agglomerates (reducedmixture) are completely melted and separated into metal and slag. Theatmospheric temperature of the moving hearth furnace 15 is varied in twosteps in the above procedure but may not be varied, and the agglomerates(mixture) 14 may be heated to an atmospheric temperature ranging from1100° C. to 1500° C. in one step, whereby the agglomerates (mixture) 14are melted and reduced at the same time. The latter procedure providesthe reduced melt in a shorter time. Both the metal and slag may bemelted and only either one may be melted. For example, only the metalmay be melted, whereby the metal is isolated from the slag.

[Solidifying Step]: The reduced melt is cooled to about 1000° C. in oroutside the moving hearth furnace 15, whereby the reduced melt issolidified into the reduced solid 16. Examples of a cooling/solidifyingunit placed in the moving hearth furnace 15 include the radiant coolerand coolant sprayer described in the first embodiment. In order to cooland solidify the reduced melt discharged from the moving hearth furnace15, a method such as water granulation may be used.

[Separating Step]: The reduced solid is separated into the metal(ferronickel) 18 and the slag 19 with the screen 17. Metal componentsmay be recovered from the resulting slag 19 by a method such as amagnetic separation process or a floatation process according to needs.The resulting metal 18 is then additionally refined according to needs,whereby commercial ferronickel is produced. Alternatively, the metal 18may be used as a semi-product (feedstock for ferronickel production),which is fed into a smelting furnace. In comparison between thesemi-products of the functions as embodiments, the reduced agglomerates(reduced mixture) of the first embodiment contain slag components butthe metal 18 of the second embodiment does not contain such slagcomponents. Therefore, in the second embodiment, energy consumed inmelting the slag components in the smelting furnace is not necessary,whereby energy consumption of the smelting furnace is greatly reduced.Furthermore, the semi-product has reduced weight depending on the weightof the removed slag components and the storage cost and transportationcost thereof can therefore be reduced; hence, it is preferable to use aprocess of the present invention in areas where nickel oxide ore isproduced. For convenience of storage and transportation, agglomerationor the like may be performed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart showing steps of producing ferronickel accordingto a first embodiment of the present invention.

FIG. 2 is a graph showing the relationship between the Ni metallizationdegree and Fe metallization degree of a reduced mixture when the contentof Ni in ferronickel is 16% by mass.

FIG. 3 is a graph showing the relationship between the Ni content of afeedstock and the Fe metallization degree and the relationship betweenthe Fe content thereof and the Fe metallization degree when the contentof Ni in ferronickel is 16% by mass and the Ni metallization degree of areduced mixture is 90%.

FIG. 4 is a flow chart showing steps of producing ferronickel accordingto another embodiment of the present invention.

FIG. 5 is a graph showing the relationship between the retention timeand the Ni metallization degree and the relationship between theretention time and the Fe metallization degree when the atmospherictemperature is 1200° C.

FIG. 6 is a graph showing the relationship between the retention timeand the Ni metallization degree and the relationship between theretention time and the Fe metallization degree when the atmospherictemperature is 1300° C.

FIG. 7 is a graph showing the relationship between the retention timeand the Ni metallization degree and the relationship between theretention time and the Fe metallization degree when the atmospherictemperature is 1200° C.

FIG. 8 is a graph showing the relationship between the supply of carbon(%) and the Ni metallization degree and the relationship between thesurplus of carbon (%) and the Fe metallization degree when theatmospheric temperature is 1200° C.

FIG. 9 is a graph showing the relationship between the heatingtemperature and the Ni metallization degree and the relationship betweenthe heating temperature and the Fe metallization degree when theretention time is 15 minutes.

REFERENCE NUMERALS

1 and 11: feedstock containing nickel oxide and iron oxide

2 and 12: carbonaceous reductant

3 and 13: agglomerator

4 and 14: agglomerates (mixture)

5 and 15: moving hearth furnace

6: reduced agglomerates (reduced mixture)

7: smelting furnace

8 and 18: metal (ferronickel)

9 and 19: slag

16: reduced solid

17: screen

BEST MODE FOR CARRYING OUT THE INVENTION Example 1

In order to assess the reduction state of a feedstock mixture smelted ina moving hearth furnace according to the present invention, a reductionexperiment described below was performed using a small-sized furnace forlaboratory use.

A feedstock, containing nickel oxide and iron oxide, having thecomposition shown in Table 2 was mixed with a carbonaceous reductantcontaining coke powder (a content of fixed carbon: 77.7 mass percent),the ratio of the feedstock to the carbonaceous reductant being 85.7 to14.3 on a mass basis. A certain amount of water was added to the mixtureand the resulting mixture was formed into pellets having a diameter of13 mm with a small-sized disk pelletizer. Those pellets were dried, fedinto the small-sized furnace in a batch mode, and then heated andreduced in such a manner that the atmospheric temperature is maintainedconstant and the retention time is varied. The reduced pellets werechemically analyzed for composition, whereby the Ni metallization degreeand the Fe metallization degree were determined. A nitrogen atmospherewas used and the atmospheric temperature was varied in two levels: 1200°C. and 1300° C. TABLE 2 (Unit: mass percent) T. Fe FeO M. Fe T. Ni 27.53.0 5.5 2.5

FIGS. 5 and 6 each show a correlation between the retaining time(retention time) and the Ni metallization degree and a correlationbetween the retaining time and the Fe metallization degree, thosecorrelations being obtained from the reduction experiment. Thecorrelations shown in FIG. 5 were obtained at an atmospheric temperatureof 1200° C. and the correlations shown in FIG. 6 were obtained at anatmospheric temperature of 1300° C. Both figures illustrate that a Nicomponent is primarily reduced as compared with a Fe component.Furthermore, the reduction rate determined at 1300° C. is greater thanthat determined at 1200° C. For example, FIG. 5 illustrates that the Nimetallization degree reaches about 90% and the Fe metallization degreeremains at about 60% when the atmospheric temperature is 1200° C. andthe retaining time (retention time) is 2 minutes. Thus, a semi-productin which the Ni metallization degree has been maximized and the Femetallization degree has been minimized can be obtained by appropriatelyadjusting the retention time depending on heating conditions such as theatmospheric temperature. Since the reduction state of the feedstockmixture in an actual moving hearth furnace is affected by the differencein heating rate due to the shape or size of the furnace and the gascomposition of the atmosphere, the optimum retention time can bedetermined by actually measuring the Ni metallization degree and the Femetallization degree, with the retention time varied in the movinghearth furnace.

Example 2

A mixture consisting of 94 parts by mass (dry basis) of nickel oxide oreand 6 parts by mass (dry basis) of coal was formed into briquetteshaving a volume of 5.5 cm³ with a briquette press. The nickel oxide orehad a total Ni content of 2.4%, a total Fe content of 14.7%, a SiO₂content of 35.5%, and an MgO content of 25.8% and the coal had a fixedcarbon content of 74.0%, a volatile matter content of 15.5%, and an ashcontent of 10.5% on a mass basis. The briquettes were fed into a rotaryhearth furnace and then reduced into a semi-product (reduced briquettes)at an atmospheric temperature ranging from 1100° C. to 1300° C. with aretention time of 5 minutes, the Fe metallization degree of thesemi-product being about 60%. In the above operation, the yield of thesemi-product (reduced briquettes) obtained from the rotary hearthfurnace was 88 parts by mass, the Ni metallization degree of thesemi-product being about 98%.

The semi-product (reduced briquettes) maintained at 1000° C. was fedinto an electric furnace and then smelted, whereby 11 parts by mass ofcrude ferronickel having an Ni content of 20% to 21% by mass and 80parts by mass of slag having an FeO content of about 10% by mass wereobtained. The electric consumption of the electronic furnace was 13000kWh per ton of Ni. This value is less than that of a known electricfurnace process (selective reduction process) using a rotary kiln as apre-reducing furnace. In the process, the electric consumption is about20000 kWh per ton of Ni.

Example 3

The same feedstock and carbonaceous reductant as those used in theExample 2 were used. A mixture consisting of 93 parts by mass (drybasis) of the nickel oxide ore and 7 parts by mass (dry basis) of coalwas mixed with a certain amount of water, and the resulting mixture wasformed into pellets having a diameter of 18 mm with a disk pelletizer.The pellets were dried with a dryer, fed into a rotary hearth furnace,and then heated and reduced at an atmospheric temperature ranging from1300° C. to 1350° C., whereby a Ni component was substantiallycompletely metallized. After the Fe metallization degree reached about60%, the resulting pellets were heated at an atmospheric temperatureranging from 1350° C. to 1450° C., whereby the pellets were melted.

The melt was then cooled and solidified with a chill plate (radiantcooler) placed in the rotary hearth furnace, and the solid wasdischarged from the rotary hearth furnace and then separated into metal(crude ferronickel) and slag with a screen. As a result, the followingcrude ferronickel and slag were obtained: 11 parts by mass of the crudeferronickel having a Ni content of 20%, a Fe content of 74%, and a Ccontent of 2% and 77 parts by mass of the slag having a FeO content of10% on a mass basis.

In the Examples 2 and 3, no binder was used for agglomeration; however,an appropriate binder may be used when the agglomerates have aninsufficient strength.

Example 4

A mixture consisting of 96.5 parts by mass (dry basis) of nickel oxideore that is difficult in reducing and 3.5 parts by mass (dry basis) ofcoal was formed into tablets having a diameter of 25 mm with a tabletpress. The nickel oxide ore had a total Ni content of 2.1%, a total Fecontent of 18.8%, a SiO₂ content of 35.0%, and an MgO content of 19.5%and the coal had a fixed carbon content of 72%, a volatile mattercontent of 18%, and an ash content of 10% on a mass basis. The tabletswere fed into a rotary hearth furnace and then reduced at an atmospherictemperature of 1200° C. FIG. 7 shows a correlation between the retainingtime (retention time) and the Ni metallization degree and a correlationbetween the retaining time and the Fe metallization degree, thosecorrelations being obtained from the reduction experiment.

The experiment shows that the Ni component is more rapidly reduced ascompared with the Fe component. Since the nickel oxide ore used in theexperiment is difficult in reducing, the Ni metallization degree issaturated when it is reached to about 56%. However, if the ore is heatedand reduced at 1200° C. for 6 minutes or more, the Ni metallizationdegree is 40% or more and the Fe metallization degree is at least 15%less than the Ni metallization degree.

Example 5

A mixture was prepared in such a manner that the same feedstock and coalas those used in Example 4 were used but the blend ratio of the nickeloxide ore to the coal was varied, and the mixture was formed intotablets having a diameter of 25 mm. The tablets were fed into a rotaryhearth furnace and then reduced at an atmospheric temperature of 1200°C. for 12 minutes (retention time), whereby the difference between theNi metallization degree and the Fe metallization degree was determined.The results are shown in FIG. 8.

The results indicate that a decrease in “the surplus of carbon (%)”specified below causes a decrease in the Fe metallization degree. Whenthe surplus of carbon is small, the reduced tablets have high strength(for example, crushing strength) and are therefore easy in handling andthe melting yield is high.

-   -   the surplus of carbon (%) =(the mass percentage of carbon in an        unreduced mixture)−(the mass percentage of oxygen bonded to Fe        or Ni in the unreduced mixture)×12/16

Example 6

The same feedstock and coal as those used in Examples 4 and 5 were used.A mixture consisting of 90.5 parts by mass of the nickel oxide ore and9.5 parts by mass of the coal was formed into tablets having a diameterof 25 mm. The tablets were fed into a rotary hearth furnace and thenreduced at an atmospheric temperature of 1200° C., 1250° C., or 1300° C.for 15 minutes, whereby the difference between the Ni metallizationdegree and the Fe metallization degree were determined. The results areshown in FIG. 9.

The results indicate that an increase in temperature causes a decreasein difference between the Ni metallization degree and the Femetallization degree. However, considering these results and the resultsof the above examples, even if the heating and reduction temperature is1300° C., the Fe metallization degree can be adjusted to a value that isat least 15% less than the Ni metallization degree by decreasing theretention time and/or controlling the content of the reductant(controlling the surplus of carbon).

INDUSTRIAL APPLICABILITY

As described above, the present invention provides a process that isuseful in producing ferronickel having a high Ni content at low costwith high efficiency and reproducibility even if a low-grade feedstockcontaining nickel oxide is used.

1. A process for producing a ferronickel, comprising: mixing a feedstockcomprising nickel oxide and iron oxide with a carbonaceous reductant toprepare a mixture; heating and reducing the mixture in a moving hearthfurnace to prepare a reduced mixture; and smelting the reduced mixturein a smelting furnace to prepare the ferronickel.
 2. The process forproducing a ferronickel according to claim 1, wherein the retention timeof the mixture placed in the moving hearth furnace is adjusted such thatthe Ni metallization degree of the reduced mixture is 40% or more andthe Fe metallization degree of the reduced mixture is at least 15% lessthan the Ni metallization degree thereof.
 3. The process for producing aferronickel according to claim 2, wherein the Ni metallization degree is85% or more.
 4. The process for producing a ferronickel according toclaim 1, further comprising cooling the reduced mixture to a temperatureranging from 450° C. to 1100° C. in the moving hearth furnace tomaintain the reduced mixture at that temperature for 17 seconds or moreor discharging the reduced mixture from the moving hearth furnace toplace the reduced mixture in another vessel to cool the reduced mixtureto a temperature ranging from 450° C. to 1100° C. in the vessel tomaintain the reduced mixture at that temperature for 17 seconds or more.5. A process for producing a ferronickel, comprising: mixing a feedstockcomprising nickel oxide and iron oxide with a carbonaceous reductant toprepare a mixture; heating and reducing the mixture in a moving hearthfurnace to prepare a reduced mixture wherein the Ni metallization degreeis 40% or more and the Fe metallization degree is at least 15% less thanthe Ni metallization degree to heat and melt the reduced mixture toprepare a reduced melt; cooling the reduced melt in the moving hearthfurnace to prepare a reduced solid or discharging the reduced melt fromthe moving hearth furnace to cool the reduced melt to prepare a reducedsolid; and separating the reduced melt into a metal and a slag toprepare the ferronickel.
 6. The process for producing a ferronickelaccording to claim 5, wherein the Ni metallization degree is 85% ormore.
 7. A process for producing a feedstock for ferronickel production,comprising: mixing a composition comprising nickel oxide and iron oxidewith a carbonaceous reductant to prepare a mixture; and heating andreducing the mixture in a moving hearth furnace to prepare the feedstockfor ferronickel production wherein the Ni metallization degree is 40% ormore and the Fe metallization degree is at least 15% less than the Nimetallization degree.
 8. The process for producing a feedstock forferronickel production according to claim 7, wherein the Nimetallization degree is 85% or more.